单项填空
从每题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21. People are living in ___ world where mouse clicks rule our life, so online searches have become _____ must.
A. the; / B. /; a C. a; a D. a; the
22. — You look tired.
— Yes. I ___ non-stop until 8 o’clock.
A. had worked B. work C. have worked D. was working
23. Most of the roads were covered with thick snow, ____ made our journey more difficult.
A. one B. it C. which D. that
